In the fields such as medical and pharmaceutical sciences and gene engineering, a centrifugal precipitator that is a centrifuge is used to process by, for example, sedimentation isolation in this taking a sample of liquids or a mixture of solids and liquids as a sample. A centrifuge has installed therein a rotor to which a container, such as tube or bottle, in which samples such as culture broth or blood is accommodated is loaded. The rotor is detachably loaded to a rotation axis that protrudes into a rotor chamber (rotary chamber) of a storage container. The rotor is driven to rotate with a driving device such as electric motor. Upon a centrifuging process on a sample in the storing container, the rotor is rotated at a high speed in the state where the sample is retained by the rotor.
The centrifuge in which a maximum speed of rotation of a rotor is set at about from 10,000 to 30,000 rpm is often used to process a sample while putting its rotor chamber at an atmospheric pressure. When a rotor is rotated in this manner in which air exists in the rotor chamber, heat of friction of air and the rotor generated during the rotation of the rotor may be bigger and it might rise the temperature of the sample. Therefore, a cooling apparatus is often mounted in a centrifuge. As the cooling apparatus, for example, a refrigerator (freezing machine) in which a cooling medium is circulated in a cooling pipe that is wound around a storing container is used as described in Japanese Patent Application Laid-Open Publication No. H01-218651.
In a centrifuge in which a cooling apparatus is mounted, operating conditions are set by inputting them by a user via an input-operation panel of the centrifuge. There are operation conditions such as a rotation speed of a rotor, that is, number of rotation, operation time of the centrifuge, that is, processing time, set temperature of the rotor, that is, cooling temperature, acceleration gradient upon start-up of the rotor, deceleration gradient upon stopping deceleration of the rotor, and so forth.
When subjecting a sample to a centrifugal process, a rotor to which the sample is loaded is attached to a rotation axis to set the rotor in a rotor chamber. After setting the rotor, an operator closes a door provided to the centrifuge and pushes a start switch on an operation panel and then the rotor is activated and rotation is started. As the rotor is accelerated, when the speed reaches the set rotation speed, the rotor is operated by a constant-velocity drive at a steady speed. When a set operation time is elapsed as the steady-speed operation of the rotor is continued, the rotation of the rotor is decelerated and the rotor is stopped. Thereafter, the user opens the door to get the rotor out of the centrifuge and get the sample after the centrifugal process of the rotor.
The refrigerator used as a cooling apparatus cools the rotor chamber by driving a motor of a compressor for sending out a coolant to circulatory supply the coolant in a cooling pipe. The compressor used in the centrifuge is normally operated at a steady speed at a commercially used power frequency, that is, 50 Hz or 60 Hz. The rotation control of the compressor is generally performed in the following manner. First, the compressor is driven until the rotor is cooled down to a set temperature and when the temperature of the rotor reaches the set temperature, the compressor is stopped. When the temperature starts to rise as heat is generated from the rotor due to friction with the air etc., the compressor is driven again.
Variety of rotors are loaded on one centrifuge and the most optimum one of the variety of rotors is selected depending on the sample to be subjected to a centrifugal process and/or separating conditions. The operating conditions of the centrifuge differ depending on the selected rotor. A rotation speed of the rotor to be set, that is, the number of rotation are various from a high speed to a low speed, and through all the conditions, the centrifuge is required to cool the rotor at a set temperature. As the rotor itself generates heat due to heat of friction with the air caused by rotation of the rotor as mentioned above, a difference is made between the temperature of the rotor and the temperature of the rotor chamber that is detected by a temperature sensor provided inside the rotor chamber. Generally, the temperature of the rotor is higher than the temperature of the rotor chamber. Thus, to maintain the rotor at the set temperature, a target controlled temperature is set including corrected temperature difference between the temperature of the rotor and the temperature of the rotor chamber and the temperature of the rotor chamber is controlled so as to obtain the target controlled temperature.
The amount of heat generation, that is, windage loss of the rotor is increased as the rotation speed of the rotor is increased. Particularly, as to windage loss at the maximum rotation speed of the rotor, an increase of the amount of heat generation of the rotor is significant upon operating the centrifuge at a set rotation speed that is higher than or equal to 48% of the maximum rotation speed. The larger the amount of heat generation of the rotor, the higher the temperature of the rotor itself, and it makes the temperature difference of the rotor and the rotor chamber larger and also the corrected amount becomes larger. Thus, the higher the rotation speed of the rotor during operation is, the more the target controlled temperature of the rotor chamber is set to be significantly lower than the set temperature of the rotor.
As methods of decelerating a rotor in a centrifuge, there are: decelerating at the maximum capacity; free-run (natural deceleration) deceleration control for decelerating only by the resistance of windage loss generated in the rotor or mechanical loss inside a motor without braking by the motor; and slow deceleration control for slowly decelerating taking a long time with setting a deceleration gradient. The latter two of the methods are used when separating a sample in which a pellet (solid matter having a heavy specific gravity) being settled out at the bottom portion of a sample container is prone to go up into a supernatant liquid. Upon finishing a centrifugal process, in the case of performing the decelerating stop control like the free-run deceleration control or the slow deceleration control, the higher the rotation speed or the larger the volume of the rotor is, the more the deceleration takes time. When decelerating from the setting rotation speed that is 48% of the maximum rotation speed taking time, the rotor is accommodated for a long time in the rotor chamber that is cooler than the set temperature since the target controlled temperature is set at a lower temperature than the set temperature. When the rotation speed of the rotor is lowered while this state is being kept, the amount of heat generation of the rotor is gradually decreased. However, as a centrifuge not mounting a heating apparatus cannot raise the temperature of the rotor chamber, the temperature of the sample loaded in the rotor is considerably lower than the set temperature, resulting in excessive cooling (icing) of the sample. When excessive cooling happens, the quality of the centrifugal process of the sample is lowered.
A preferred aim of the present invention is to provide a centrifuge capable of preventing lowering of process quality of the sample even when the rotation of the rotor is slowly decelerated with taking time upon stopping a centrifugal process.